1. Field of the Invention
The present invention relates generally to a method and apparatus for isolating mechanical vibrations in a structure or body which is subject to harmonic or oscillating displacements or forces, and is of particular utility in the field of aircraft, in particular helicopters.
2. Description of the Prior Art
For many years, effort has been directed toward the design of apparatus for isolating a vibrating body from transmitting its vibrations to another body. Such apparatus are useful in a variety of technical fields in which it is desirable to isolate the vibration of an oscillating or vibrating device, such as an engine, from the remainder of the structure. Typical vibration isolation and attenuation devices ("isolators") employ various combinations of the mechanical system elements (springs and mass) to adjust the frequency response characteristics of the overall system to achieve acceptable levels of vibration in the structures of interest in the system. One field in which these isolators find a great deal of use is in aircraft, wherein vibration-isolation systems are utilized to isolate the fuselage or other portions of an aircraft from mechanical vibrations (such as harmonic vibrations) which are associated with the propulsion system and which arise from the engine, transmission, and propellers or rotors of the aircraft.
Vibration isolators are distinguishable from dampening devices in the prior art that are erroneously referred to as "isolators." A simple force equation for vibration is set forth as follows: EQU mx+cx+kx=F
A true vibration isolator utilizes acceleration of a fluid body (mx) to cancel the displacement of vibration (kx). A dampening device is concerned with restricting flow of a fluid or other body and thus velocity (cx), and does not cancel vibration, but merely absorbs its frequency.
One important engineering objective during the design of the aircraft vibration-isolation system is to minimize the length, weight and overall size including cross-section of the isolation device. This is a primary objective of all engineering efforts relating to aircraft. It is especially important in the design and manufacture of helicopters, which are required to hover against the dead weight of the craft and which are thus somewhat constrained in their payload in comparison with fixed-wing aircraft.
Another important engineering objective during the design of vibration-isolation systems is the conservation of the engineering resources that have been expended in the design of other aspects of the aircraft or in the vibration-isolation system. In other words, it is an important industry objective to make incremental improvements in the performance of vibration isolation systems which do not require radical re-engineering or complete redesign of all of the components which are present in the existing vibration-isolation systems.
A marked departure in the field of vibration isolation, particularly as applied to aircraft and helicopters, is disclosed in commonly assigned U.S. Pat. No. 4,236,607, which is entitled "Vibration Suppression System," which issued on Dec. 2, 1980, to Halwes, et al., and which is incorporated herein by reference This patent discloses a vibration isolator in which a dense, low-viscosity fluid is used as the "tuning" mass to counterbalance (or cancel) oscillating forces transmitted through the isolator. This isolator employs the principle that the acceleration of an oscillating mass is 180 degrees out of phase with its displacement.
Halwes, et al. recognized that the inertial characteristics of a dense, low-viscosity fluid, combined with a hydraulic advantage resulting from a piston arrangement, could harness the out-of-phase acceleration to generate counter-balancing forces to attenuate or cancel vibration. This invention provided a much more compact, reliable, and efficient isolator than was provided in the prior art. The original dense, low-viscosity fluid contemplated by Halwes was mercury, which is toxic and highly corrosive. Since Halwes' early invention, much of the effort in this area has been directed toward replacing mercury as a fluid or to varying the dynamic response of a single isolator to attenuate differing vibration modes. An example of the latter is found in commonly assigned U.S. Pat. No. 5,439,082, entitled "Hydraulic Inertial Vibration Isolator" which was issued on Aug. 8, 1995 to McKeown, et al.
Several factors affect the performance and characteristics of the Halwes-type isolator, including the density and viscosity of the fluid employed, the relative dimensions of components of the isolator, and the like. Improvements in the design of such isolators that permit reduction in their size and weight, without sacrificing their ability to attenuate or cancel vibration, are needed.